1. Field of the Invention
The present invention relates to an organometallic compound having an excellent luminous efficiency and capable of emitting light of spectrum components of red or blue wavelength band, and to a light emitting device such as organic electroluminescent device using the same.
2. Related Art
As the recent development of information technology (IT), there have been increased needs for low-profile display devices having a small thickness of about several millimeters and capable of achieving full color display. Approaches to realize the full color display can be generally classified into three types of methods: (1) arranging a number of light emitting devices which respectively emit monochromatic light of red, green and blue which are the primary colors of light; (2) using white light emitting devices in combination with color filter which allow monochromatic light of primary colors to pass through; and (3) using light emitting devices which emit ultraviolet light or blue light in combination with wavelength converting means for converting the above light into monochromatic light of the primary colors of light.
As such a display apparatus which is capable of full-color display according to the above method (1), display apparatuses using a electroluminescent device are known. As such an electroluminescent device, inorganic electroluminescent devices using inorganic substances such as sulfates as a light emitting material. Since an inorganic electroluminescent device requires high driving voltage and hence requires AC driving, it becomes difficult to increase the reliability of peripheral driving circuits and hence the cost will increase. Furthermore, since it is driven by high AC voltage, strong electromagnetic waves are radiated, which posed a problem that an adverse effect could be exerted on peripheral electronic devices.
In recent years, for realizing low-profile light emitting devices wherein the above-mentioned problems are solved, organic electroluminescent devices using amorphous thin films made of organic substances as light emitting materials have been enthusiastically developed.
It is generally considered that a green light emitting device can be realized in a relatively easy manner with high performance, while it is difficult to realize a blue or red low-profile light emitting device. Also, in the cases of organic electroluminescent devices, devices that can emit light of spectrum components of a blue or red wavelength band are desired.
Forrest, Stephen R. et al. disclosed in Appl. Phys, Lett., 1999, 75(1), 4–6, an organic electroluminescent device using a mixture light emitting layer in which tris(2-phenylpyridine) iridium (Ir(ppy)), which is green iridium (Ir(ppy)), which is green phosphorescence luminescent substance, is mixed into 4,4′-bis(carbazole-9-yl)-biphenyl (CBP) in a concentration of 1 to 12% wt. In such a light emitting device, a light emission peak which is considered as coming from the triplet excited state of Ir(ppy) is observed, and an excellent luminous efficiency is obtained.
Those capable of emitting light from the triplet excited state as described above emit light via the triplet excited state that is not usually involved in light emission and thus is not utilized effectively, so that the luminous efficiency can be significantly improved. For this reason, also for red or blue light emission, organic electroluminescent devices that are capable of emitting light via the triplet excited state even are requested.